Vol. 65.] GLACIAL EROSION IN NORTH WALES. 291 



where I saw the uplands in 1900, the general altitude is 1400 or 

 1500 feet. This would indicate that Snowdon and its high neigh- 

 hours had a relief of some 2000 feet over the upland plain, and 

 hence would fully warrant their being classed as monadnocks, as 

 already suggested. 



IX. Dissection op the Uplifted Plain. 



Ramsay had a clear idea of the consequences that must follow 

 the uplift of a plain of marine denudation. He wrote, for ex- 

 ample :— 



' In the long, gentle slope of the country from the hills on the right bank of 

 the Teifi to the sea, we observe the effects first of the planing away of surface 

 irregularities during an average general depression, and again of the further 

 scooping out of valleys in this plain during a subsequent average elevation . . . 

 the original approximate levels being reduced to a more fragmentary condition 

 by the after denudations that scooped out their intersecting valleys ' (1846, 

 p. 330). 



But, although the general principles involved were thus long ago 

 recognized, their more precise application has hardly been attempted, 

 and the degree of development of the Welsh valleys before the 

 coming of the Ice Age has still to be investigated. 



The stage reached in the dissection of the Welsh upland plain 

 when the Glacial Period supervened is of difficult determination, so 

 long as the amount of erosion accomplished during the Glacial 

 Period is unknown. If it be concluded that the erosion of the 

 Glacial Period was of small amount, then the mature forms of the 

 valleys of to-day must be credited to pre-Glacial erosion ; but, if it 

 be believed that the erosion of the Glacial Period was of large 

 amount, then the mature width of the existing valleys may be 

 ascribed to glacial action, and before this action began the valleys 

 may have been narrow. The best means that I have come upon 

 for reaching a reasonable opinion on this matter,.independent of all 

 estimates of glacial erosion, involves the postulate that Wales and 

 Devon -Cornwall have had — except in respect to glaciation — a 

 similar history in modern geological times. A paragraph must be 

 allowed for the presentation of the grounds of this postulate. 



The chief reasons for regarding the modern geological history of 

 these provinces as similar are, first, the similarity of the relation 

 in which they stand to the Mesozoic formations which overlap them 

 unconformably on the east ; and second, the similarity of the 

 topographic forms — apart from those of the glaciated districts of 

 Wales — that have been developed in them. It is believed that the 

 essential features of the two provinces in both these respects are 

 summarized in fig. 1 (p. 292), in which the compound mass of ancient 

 and Mesozoic rocks was reduced in a Tertiary cycle of normal 

 erosion to a wide-spread peneplain, at the level T, surmounted by 

 occasional monadnocks, M, and then uplifted and again dissected in 

 a later cycle of erosion ; a new peneplain, L (dotted), was then 



